Differential transmission is a transmission method that causes currents with opposite phases to flow in two signal lines and transmits signals using the potential difference between the signal lines. The differential transmission has advantages of high-speed data transmission, high tolerance to noise, and low emission of electromagnetic interference (EMI) noise, compared with single-ended transmission.
In the differential transmission, if phases of two signals, pulse widths of the signals, or amplitudes of the signals are shifted from each other, an unbalanced component that is called a skew may occur. The unbalanced component causes the occurrence of a noise current called a common mode current and causes an erroneous operation of a system. Thus, a differential transmission line has a configuration for suppressing the occurrence of an unbalanced component.
For example, a technique for suppressing undesired mode conversion in a curved region of a transmission line has been proposed. According to this technique, the differential transmission line has a board, a grounded conductive layer formed on the top side of the board, and first and second signal conductors arranged side by side on the bottom side of the board. The first signal conductor and the grounded conductive layer form a first transmission line. The second signal conductor and the grounded conductive layer form a second transmission line. The differential transmission line is formed of the first transmission line and the second transmission line. The differential transmission line includes a curved region. Both ends of the curved region are connected to straight regions. It is assumed that the width of the signal conductor in the curved region is Wb1, the width of the second signal conductor in the curved region is Wb2, the width of a gap between the first signal conductor and the second signal conductor is Gb, the width of the first signal conductor in the straight regions is Ws1, and the width of the second signal conductor in the straight regions is Ws2. In addition, it is assumed that the width of a gap between the first signal conductor and the second signal conductor is Gs, a minimum distance between the center of the curvature of the curved region and a line edge, located on the side of the center of the curvature of the first signal conductor, of the first signal conductor in the curved region is Rb1, and a distance between the center of the curvature of the curved region and a normal to a straight line extending from a line edge, located on the side of the center of the curvature of the first signal conductor, of the first signal conductor in the straight regions is Rs1. According to this technique, undesired mode conversion in the curved region of the transmission line is suppressed by setting these the values Wb1, Ws1, Wb2, Ws2, Gb, Gs, Rb1, and Rs1 so as to ensure that Wb1 is smaller than Ws1, Wb2 is smaller than Ws2, Gb is smaller than Gs, and Rb1 is larger than Rs1.
In addition, the following technique has been proposed: a technique for suppressing a reduction, caused by the unevenness of differential impedance of a differential transmission line, in the quality of transmission in a printed board in which the differential transmission line composed of a first wiring and a second wiring and an element connected to the differential transmission line are mounted in a wiring layer that is in contact with a dielectric layer. According to this technique, the width of the first wiring and the width of the second wiring are gradually reduced to a predetermined width on the side of one of ends of each of the first and second wirings, and the first and second wirings are axisymmetric in a region in which the widths of the first and second wirings are gradually reduced.
International Publication Pamphlet No. 2007/000934 and Japanese Laid-open Patent Publication No. 2005-340506 are examples of related art.
FIG. 1 is a diagram illustrating an example of a wiring pattern of a pair of signal lines that form a differential transmission line. A differential transmission line 100 includes a first signal line 101 and a second signal line 102. The first and second signal lines 101 and 102 are curved at an angle of 90° in a curved region 103. The second signal line 102 is arranged on the inner side of the curved region 103 and is a meander line curved and extending away from the first signal line 101 and the difference between the lengths of the first and second signal lines 101 and 102 is reduced. If the difference between the lengths of the first and second signal lines 101 and 102 is large, the difference between the phases of two signals transmitted in the first and second signal lines 101 and 102 may occur and cause the occurrence of an unbalanced component. It is, therefore, preferable that the difference between the lengths of the first and second signal lines 101 and 102 be small.
However, if the differential transmission line includes the meander line, the area of a region occupied by the signal lines may be large and it may be difficult to design a pattern of the differential transmission line. FIG. 2 is a diagram comparing a region (illustrated on the upper side) occupied by differential transmission lines including meander lines with a region (illustrated on the lower side) occupied by differential transmission lines without a meander line. In an example illustrated in FIG. 2, differential transmission lines 100A to 100D that include meander lines are arranged side by side, form four signal line pairs, and extend in a direction intersecting with a direction in which signal lines extend, and differential transmission lines 100E to 100H without a meander line are arranged side by side, form four signal line pairs, and extend in the direction intersecting with the direction in which the signal lines extend. The differential transmission lines 100A to 100D that form the signal line pairs have meander portions 101A to 104A included in signal lines of the signal line pairs and curved and extending away from the other signal lines of the signal line pairs. It is assumed that inter-wiring distances (wiring pitches) L between the signal lines of the differential transmission lines 100A to 100D including the meander lines and inter-wiring distances (wiring pitches) L between the signal lines of the differential transmission lines 100E to 100H without a meander line are equal to each other and that distances M between adjacent differential transmission lines are equal to each other. As illustrated in FIG. 2, a region R1 in which the differential transmission lines 100A and 100D that form the four pairs and include the meander lines are arranged is larger than a region R2 in which the differential transmission lines 100E to 100H that do not include a meander line are arranged. In addition, as illustrated in FIG. 3, if the differential transmission lines 100A and 100D that form the four pairs are curved, the area of the region occupied by the differential transmission lines 100A and 100D is further increased.
As described above, if a differential transmission line includes a meander line, the area of a region occupied by signal lines of the differential transmission line is increased. In order to suppress the increase, it is considered that the widths of curves of the meander line are reduced. In this case, however, the number of the curves is increased in order to reduce the difference between the length of the meander line and the length of another line of the differential transmission line. If a sufficient number of curves are not secured, the difference between the lengths may not be sufficiently reduced.